U.S. Pat. No. 5,454,490 entitled "CONICAL INSERTS FOR CHIP BIN" discloses a method and apparatus for improving the flow of wood chips through a vertical wood chip treatment vessel by introducing conical rings to the interior surface of the vessel. These frustum-shaped rings provide relief of compaction forces due to the weight of the material and promote the uniform movement and treatment of the chips in the vessel (typically referred to as a "chip bin"). However, the patent is primarily directed to treating chips in a relatively dry state, where bridging or compaction may occur. No liquids are typically present in such bins, though one form of treatment disclosed is the introduction of steam to the bin.
In copending U.S. patent applications Ser. No. 08/936,047 filed on Sep. 23, 1997 now U.S. Pat. No. 5,985,096 and Ser. No. 08/953,880 filed on Oct. 24, 1997 now U.S. Pat. No. 6,129,816 the movement through and treatment of cellulose material in vertical continuous digesters is discussed. In particular, these applications (the disclosures of which are incorporated by reference herein) discuss the various degrees of compaction that can occur in a vertical column of comminuted cellulosic fibrous material (for example, wood chips) and how this compaction affects both the movement of the column and the treatment of the material with treatment liquids, especially in the vicinity of the annular screen assemblies that are so common to vertical continuous cooking vessels. The various embodiments disclosed in these applications show how the movement and treatment can be made more uniform by introducing novel screen assembly geometries. The present invention also improves the uniformity of the movement of material in treatment vessels and thus improves the uniformity of the treatment.
Surprisingly, according to the invention it has been discovered that the vertical movement of comminuted cellulosic fibrous material slurries, such as wood chips slurries, can be improved (e.g. made more uniform) by providing protrusions on the internal substantially vertical surface of the cooking vessel. These protrusions may be substantially continuous, that is, substantially annular, or may be intermittently distributed about the circumference of the internal surface of the vessel. In one embodiment, the protrusions consist of or comprise one or more substantially annular rings. In another embodiment, the protrusions consist of or comprise one or more rows of two or more individual structures evenly distributed or non-evenly distributed about the circumference of the internal surface of the vessel. The protrusions (also called protuberances) may be aligned in one or more rows, or one row of protuberances may be off-set from an adjacent row. One or a plurality of rings or rows may be used in any part of a treatment vessel, for example, a digester, where column movement may be a problem. This includes in the top of the vessel, in the middle of the vessel, and in the bottom of the vessel.
Though the mechanism by which these protuberances promote the uniform movement of the cellulose material is uncertain, it is believed that the protuberances provide a temporary relief of the compressive forces within the column and thus the temporary relief of the normal forces on the internal surface of the vessel so that friction between the chip mass and the vessel wall is temporarily decreased or substantially eliminated. It is believed that this reduction in friction permits the cellulose material to more uniformly flow through the treatment vessel and allow more uniform treatment of the material.
The protuberances or substantially annular rings of the invention may take a variety of shapes or geometries. The geometry of the rings may take any form, but is typically a geometry that does not extend more than about one foot [0.3 meter] from the internal surface of the vessel [the vessel typically has an interior diameter of between about 10-40 feet]. In a preferred embodiment, the protuberances or rings do not extend more than about 4 inches [10 cm] from the vessel surface, preferably not more than about 2 inches [5 cm] from the surface. A preferred embodiment of the rings or protuberances is one having a taper in the direction of the movement of the material such that the upper internal diameter or radius of the protuberance or ring is greater than the lower internal diameter or lower internal radius of the ring or protuberance. The protuberance or ring preferably has a taper angle of about 70 degrees or less, typically 45 degrees or less, but is preferably 15 degrees or even 10 degrees or less, e.g. between about 5-15 degrees. The protuberance or ring may have a height of 3 feet [1 meter] or less, but is preferably 1 foot [0.3 meter] or less in height, e.g. about 0.5-3 feet [0.15-1 meter]. If two or more rings or rows are used, the rings or rows may be separated by 12 feet [4 meter] or more, or may be separated by only 6 feet [3 meter] or even 3 feet [1 meter], or only 1 foot [0.3 meter], e.g. between about 1-12 feet [0.3-4 meter].
The one or more rings or protuberances may be made from any appropriate material, for example, carbon steel or stainless steel or a nickel alloy, or plastic, for example Dupont Teflon.RTM., or a Teflon.RTM. containing plastic, or having a polytetrafluoroethylene or like low friction material coating. They may be secured to the shell by any form of appropriate fastening, for example, they may be attached by welding, or adhesive, or by one or more mechanical fasteners (e.g. threaded fasteners).
In one embodiment of the invention, the one or more (e.g. 1-10) annular rings or protuberances are metallic and perform the dual role of enhancing column movement and providing a cathode or anode for anodic corrosion protection of the vessel.
According to one aspect of the present invention there is provided a method of treating a liquid slurry of comminuted cellulosic fibrous material (e.g. at about 8-20% consistency) in a substantially vertical vessel having an internal surface, comprising: (a) Introducing the slurry into the vessel so that the slurry moves substantially downwardly in the vessel in a column. And, (b) at a plurality of vertically spaced locations in the vessel temporarily relieving compressive forces within the column and the normal forces on the internal surface of the vessel so that friction between the comminuted material and the vessel internal surface is temporarily decreased, or substantially eliminated, providing more uniform flow of the material in the vessel.
In the method (b) may be practiced to temporarily reduce the friction by at least 20% (e.g. 20-99%). Typically (b) is practiced by providing a plurality of vertically spaced compression-relieving surface manifestations on the internal surface of the vessel. For example, (b) is further practiced by providing at least one substantially continuous annular element having an inner surface that protrudes into the vessel from the internal surface a greater distance at a lower portion thereof than at a higher portion thereof. That is, (b) may be further practiced by providing a curved inner surface; or a sloped inner surface having an angle with respect to the vertical of between about 5-15.degree.; and/or (b) is further practiced by vertically spacing at least some manifestations between about 1-12 feet, and by providing the manifestations so that the maximum radial spacing thereof from the internal surface is between about 1-12 inches.
In one embodiment (b) is further practiced by providing at least one surface manifestation with an inner surface which contacts the slurry column of a material having low friction properties substantially the same as polytetrafluoroethylene.
The method may further comprise (c) cooking the material of the slurry in the vessel at a temperature above 90.degree. C. with a cooking liquid, e.g. by cooking the material with kraft cooking liquor at superatmospheric pressure and at a temperature above 100.degree. C. (e.g. 140-160.degree. C.).
According to another aspect of the invention there is provided: A substantially vertical vessel having an internal surface comprising: A plurality of vertically spaced sets of circumferentially discontinuous protrusions extending inwardly from the internal surface a maximum distance of about 1-12 inches. An inlet at or near a top portion of the vessel. And, an outlet at or near a bottom portion of the vessel. For example, the protrusions have an arcuate cross-section, or a substantially isosceles or scalene triangular, or rectangular cross-section. Typically the protrusions have a height of between about 1-3 feet, and a vertical spacing between at least two sets of between about 1-12 feet. For example, at least two sets of protrusions vertically spaced between about 1-12 feet have the protrusions thereof circumferentially offset from one set to the next. Also the protrusions may be circumferentially spaced from each other between about 5-30.degree., and an arcuate distance of between about 1-10 feet.
According to another aspect of the present invention there is provided a method of treating a liquid slurry of comminuted cellulosic fibrous material in a substantially vertical vessel having an internal surface, comprising: (a) Introducing the slurry into the vessel so that the slurry moves substantially downwardly in the vessel in a column. And, (b) at a plurality of vertically spaced locations in the vessel, causing the slurry to flow over surface manifestations which extend into the vessel a maximum distance of between about 1-12 inches. In the method (b) may be further practiced by providing a plurality of circumferentially discontinuous protrusions at each of a plurality of different levels within the vessel. Other details and modifications may be as described above.
According to yet another aspect of the present invention there is provided a substantially vertical vessel having an internal surface, and comprising: An inlet at or adjacent a top portion of the vessel. An outlet at or adjacent a bottom portion of the vessel. And, at least one substantially continuous annular protrusion connected to the internal surface and in a substantially horizontal plane, and having a maximum spacing from said internal surface of between about 1-12 inches, the protrusion having a cross-section selected from the group consisting essentially of right triangular, scalene triangular, isosceles triangular, arcuate, and rectangular. The protrusions may have a substantially isosceles triangular cross-section with an apex angle between about 90-175.degree., or may be arcuate in cross-section with a radius of curvature equal to or greater than its height. The vessel may further comprise a plurality of the protrusions, vertically spaced from each other between about 1-12 feet, and each having a height of between about 1-3 feet.
It is the primary object of the present invention to provide a method and vessel which effect more uniform flow (and thus typically uniform treatment) of material, such as cellulosic fibrous material slurry, e.g., in a digester. This and other objects of the invention will become clear from the detailed description of the invention and from the appended claims.